The patent of Burack et al., U.S. Pat. No. 5,259,051 ('051), granted Nov. 2, 1993, hereby incorporated by reference herein, describes a method for making optical backplanes by using a robotic routing machine to apply optical fibers to a flat surface of a flexible plastic substrate. The fibers are bonded to the substrate by a pressure-sensitive adhesive, and after routing they are covered by a thermoplastic sheet that encapsulates them to protect them, to give structural stability, and to keep the optical fibers in place during the handling of the optical backplane. The component optical fibers are typically used as large-capacity transmission lines between printed wiring boards or between optical circuits.
The Burack et al. '051 patent, the patent of Burack et al., U.S. Pat. No. 5,292,390, granted Mar. 8, 1994, and the aforementioned copending application of Burack et al., all describe the use of a thermoplastic material such as polyurethane for encapsulating or covering the optical fibers of an optical backplane. A problem with optical backplanes made from this material is that they do not meet the flammability specifications required for certain electronic equipment such as complex switching systems. Thermoplastic, by its nature, flows when heated, which may affect the structural integrity of the optical backplane. More importantly, when it flows in response, for example, to a flame, it exposes the underlying adhesive to the air, which could cause the adhesive to ignite. Present flammability requirements could be met if the encapsulant were of a material capable of withstanding a flame and a heat of two hundred degrees Centigrade without igniting or losing its structural integrity.
The above-described references describe the need for an encapsulant that stabilizes the ends of the optical fibers with great precision, and yet does not exert such a force on the fibers to break them, particularly at "crossover" locations, that is, locations at which one fiber overlaps one or more other fibers. There is therefore a continuing need for an encapsulant that will meet these requirements and yet will not be significantly structurally affected by temperatures of at least two hundred degrees Centigrade.